Jan,
your SQL statement is incorrect. Using double quotes (") indicates a column
reference, and I bet you don't have columns named "12345". If you want to
compare string columns with constant values, use single quotes (').

I.e.

Select (Sum(Menge*Preis)+Sum(Versicherungspreis)) from Orderd where
(Linknummer='12345') and (retoure='1') and (NOT (Zaehler in (Select Zaehler
from Rueckzaehler where Ordernummer='12345')))

should work (unless Linknummer, retoure or Ordernummer is defined as some
kind of number).

HTH,
Set